Polymersomes and related encapsulating membranes

ABSTRACT

The present invention provides biocompatible vesicles comprising semi-permeable, thin-walled encapsulating membranes which are formed in an aqueous solution, and which comprise one or more synthetic super-amphiphilic molecules. When at least one super-amphiphile molecule is a block copolymer, the resulting synthetic vesicle is termed a “polymersome.” The synthetic, reactive nature of the amphiphilic composition enables extensive, covalent cross-linking of the membrane, while maintaining semi-permeability. Cross-linking of the polymer building-block components provides mechanical control and long-term stability to the vesicle, thereby also providing a means of controlling the encapsulation or release of materials from the vesicle by modifying the composition of the membrane. Thus, the encapsulating membranes of the present invention are particularly suited for the reliable, durable and controlled transport, delivery and storage of materials.

GOVERNMENT SUPPORT

This work was supported in part by grants from the National ScienceFoundation, grant numbers DMR96-32598 and DMR 98-09364, and also bygrants from the Whitaker Foundation and the National Institutes ofHealth, grant numbers RO1-HL62352-01 and POI-HL18208. The government mayhave certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the preparation and use of vesicles andrelated encapsulating membranes made in aqueous solution fromamphiphilic polymers and related molecules.

BACKGROUND OF THE INVENTION

Membranes that are stable in aqueous media are heavily relied upon forcompartmentalization by biological cells. For instance, the outermostplasma membrane of a cell separates the inside of a cell from theoutside and, like most cell membranes, it is a self-assembled, complexfluid of biological molecules, primarily lipids and proteins. Only a fewmolecules, such as water and small, uncharged organic molecules,significantly permeate the membrane. A biomembrane also possessesstability and other thermo-mechanical properties that are not unrelatedto passive permeability and are certainly central to cell function (see,e.g., Lipowsky and Sackmann, Eds., Structure and Dynamics of Membranesfrom Cells to Vesicles, Handbook of Biological Physics, vol 1 (ElsevierScience, Amsterdam, 1995); Bloom et al., Q. Rev. Biophys. 24:293(1991)).

The same characteristics of permeability and thermo-mechanicalstability—in addition to biocompatibility—also affect how lipid vesiclesthat are assembled in vitro and that are also known as liposomes caneffectively encapsulate and deliver a long list of bioactive agents(Needham et al., in Vesicles, M. Rosoff, Ed. (Dekker, New York, 1996),chap. 9; Cevc & Lasic in Handbook of Biological Physics, chaps. 9-10,1995; Koltover et al., Science 281:78 (1998) Harasym et al., CancerChemother. Pharmacol. 40:309 (1997)). The typical liposome is comprisedof one or more bilayer membranes, each approximately 5 nm thick andcomposed of amphiphiles such as phospholipids. Each bilayer exists as atemperature- and solvent-dependent lamellar phase that is, in itssurface, in a liquid, gel, or liquid-gel coexisting state. Because of acertain intrinsic biocompatibility of phospholipid vesicles, many groupshave developed them for use as encapsulators and delivery vehicles.Vesicles surrounded by a lipid bilayer can range in diameter from assmall as tens of nanometers to giants of 0.5-40 microns.

Phospholipid vesicles are materially weak and environmentally sensitive.Transit through the digestive tract, for example, can expose liposomesto a host of solubilizing agents. Repeated transit through themicrocirculation can also tear apart giant phospholipid vesicles whichcannot withstand high fluid shear. Smaller phospholipid vesicles may notfragment, but they tend to adhere, and are thus cleared fromcirculation. Circulating cells suppress their own adhesion partlythrough a brushy biopolymer layer, known as the glycocalyx, which facesthe environment. The glycocalyx has, to some extent, been mimicked inliposome systems by the covalent addition to lipids of hydrophilicpolyethyleneglycol (PEG) polymer chains. To maximally extend a vesicle'scirculation lifetime (about ten hours), a suitable PEG weight rangesbetween about two and five kilograms/mole.

To further counteract mechanical forces imposed on their membranes,cells often also possess a sub-membranous network of cross-linkedproteins (Alberts et al., in Molecular Biology of the Cell, 3^(rd) ed.,pp489-493 and 800-1, Garland Publ., Inc., New York, 1999). The red bloodcell, as an example, survives repeated deformation through themicrocirculation without fragmentation, but only because it has across-linked network of peripheral membrane proteins. Without such anetwork, the cells cannot withstand such circulation for more than a fewhours even with a glycocalyx (Schmid-Schoenbein et al., Blut 52(3):131(1986)). With a normal membrane network, red blood cells circulate inhumans for more than 100 days. In terms of measurable properties, thenetwork imparts a shear elasticity that is only achievable with across-linked structure.

Past efforts to enhance the stability of lipid lamellae against shearand other factors, resulted in the synthesis of many different modifiedlipid molecules with polymerizable double bonds. Such bonds were locatedeither at the surfactant head group, or more commonly, at differentlocations on the hydrophobic tails (Fendler et al., Science 223:888(1984); Liu et al., Macromolecules 32:5519 (1996)). This approachclearly had the ability to generate covalently inter-connectedpoly-amphiphiles when reacted after self-assembly into membranes perordinary lipids. However, a fully, covalently interconnected network oflipids requires complete cross-linking of the membrane of a vesicle, andthe full extent of cross-linking achievable with cross-linkable lipidsappears to be difficult to ascertain. O'Brien's group (Sisson et al.,Macromolecules 29:8321 (1996)) has used solubility in hexafluoropropanolto estimate a degree of polymerization up to at least 1000. Thiscorresponds to a vesicle diameter of about 10 nanometers, if one assumescomplete cross-linking within and between layers of the bilayer, and atypical lipid area of about 0.5 square nanometers per lipid. Detergentinduced leakage of entrapped solutes was strongly inhibited bycross-linking. It is clear, however, that no fully cross-linked lipidvesicle larger than several hundred nanometers has been reported.

A cross-linkable amphiphile related to cross-linkable phospholipids hasbeen made by Komatsu et al., J. Am. Chem. Soc. 119:11660 (1997)).Tetrakis(aminophenyl)porphyrin contains four hydrophobic bixin sidechains that each terminate in a small hydrophilic carboxylate group andharbor approximately ten (photo)reactive double bonds along the backboneof each bixin chain. When dissolved in the organic solvent,tetrahydrofuran (THF), and rapidly injected into a one-eighth volume ofwater and sonicated, the synthetic molecules reportedly formed vesicles.However, the resulting membranes are porous. Irradiation led to what wasclaimed to be the first spherical membrane structure of molecularthickness, which was considered a single, dehydratable, balloon-shapedpolymer molecule insoluble in a predominantly organic solvent, such as95% ethanol. Electron micrographs showed spherical particles of lessthan 100 nm, while collapsed particles studied by atomic forcemicroscopy were reported to have a height of about 7 nm. Whether thecross-linked shells were truly semi-permeable vesicles or were highlyporous macromolecular shells, as Komatsu et al. suggested, leaves openthe question of whether, to date, a wholly cross-linked vesicle of anysize has actually been produced. Certainly, no cross-linked vesiclelarger than several hundred nanometers has been reported.

Small amphiphiles of natural origin, such as phospholipids have inspiredthe engineering of high molecular weight analogs, which alsoself-assemble into complex phases in aqueous media similar to thoseobserved for phospholipids. For example, vesicles have been assembled inaqueous solution by Uchegbu et al., J. Pharm. & Pharmacol. 50:453(1998)) using the naturally occurring macromolecule chitosan modified bythe covalent attachment of many fatty acid pendant groups. The resultingself-assembled vesicles were 300-600 nanometers in diameter, and wereshown to be bio- and haemocompatible. Although such modified naturalproducts have disadvantages of variability in the natural polymer and alack of precise control in covalent modification, the assembly ofmembranes from amphiphiles of high molecular weight has the potential toimprove vesicle stability. The overall approach has similarities tolipid cross-linking, but a primary distinction lies in the fact that,with cross-linking, self-assembly of the membrane must occur first.

Many wholly synthetic, amphiphilic molecules are also significantlylarger (in molecular weight, volume, and linear dimension) thanphospholipid amphiphiles, and have therefore been called“super-amphiphiles” (Cornelissen et al., Science 280:1427 (1998)).Cornelissen et al. used polystyrene (PS) as a hydrophobic fraction intheir series of synthetic block copolymers designatedPS40-b-(isocyano-L-alanine-L-alanine)y. For y=10, but not y=20 or 30,small collapsed vesicles with diameters ranging from tens of nanometersto several hundred, and a bilayer thickness of 16 nanometer werementioned as existing under a single acidic buffer condition (0.2 mMNa-acetate buffer, pH 5.6). However, bilayer filaments and superhelicalrods existed, without explanation, under the same solution conditions,thus making the stability of the collapsed vesicles, relative to theother microstructures, highly uncertain for the studied polymer.Furthermore, no demonstration of semi-permeability was reported, andreasons for apparent vesicle collapse were not given, further raisingquestions of vesicle stability.

Additional spherical shell structures smaller than a few hundrednanometers, and which required the presence of organic solvents mixedinto water to drive their formation, include those assembled fromvarious block copolymers as observed by Yu et al., Macromolecules31:1144 (1998); Ding et al., J. Phys. Chem. B 102:6107 (1998);Henselwood et al., Macromolecules 31:4213 (1998)). However, thereappears in the prior art only one example of a wholly syntheticsuper-amphiphile that has the unpredicted capacity to self-assemble inaqueous solution, albeit only under moderately acidic pH conditions,into a vesicle-like microstructure, and that is the reported work ofCornielissen et al., 1998, although even those structures were ofquestionable state and stability.

Both amphiles and super-amphiphiles can exist in a broad variety ofmicrophases and bulk phases that include not only lamellar, but alsohexagonal, cubic, and more exotic phases (see review by Lipowsky andSackmann, in Handbook of Biological Physics, 1995; Bates, Science251:898 (1991). Based on the work of Hajduk et al. (see, J. Phys. Chem.B 102:4269 (1998)), the ability of super-amphiphilic block copolymers toform lamellar phases in aqueous solutions can be regulated by bothsynthetic tuning of polymer chemistry and physical variables like, suchas concentration and temperature. Evidence has now accumulated that indilute solutions certain diblock copolymers, such aspolyethyleneoxide-polyethylethylene (PEO-PEE, wherein PEO is structuralequivalent to PEG), can form not only worm-like micelles (Won et al.,Science 283:960-3 (1999)), but also unilamellar vesicles (Discher etal., Science 284:1143 (1999)).

In addition, because of the synthetic control over molecularcomposition, properties of membranes assembled from super-amphiphilescan be controlled in novel ways. For instance, a super-amphiphilicpolymer can be made far more reactive than a much smaller phospholipidmolecule simply because more reactive groups can be designed into thepolymer. The principle was first illustrated for the aforementionedworm-like micelles in which polyethyleneoxide-polybutadiene (PEO-PBD)mesophases were successfully cross-linked into bulk materials withcompletely different properties, notably an enhanced shear elasticity(Won et al., 1999). The resulting microstructures, though assembled inwater, could withstand dehydration, as well as exposure to an organicsolvent, such as chloroform.

In the absence of cross-linking, microstructures of amphiphiles andsuper-amphiphiles are generally unstable to treatments that couldotherwise prove very useful for a range of applications that mightbenefit from, for example, sterilization, or long-term dry storage.

Despite recent advances, there remained until the present invention along felt need in the art for stable, aqueous-formed vesicles whichcould be more broadly engineered but still have demonstrable features incommon with a biomembrane or a mimic, including: biocompatibility,selective permeability to solutes, the ability to retain internalaqueous components and control their release, the ability to deform yetbe relatively tough and resilient, and the ability to extensivelycross-link within the membrane in order to withstand extremeenvironments.

SUMMARY OF THE INVENTION

The present invention meets the need in the art by providing not only anillustrative set of stable super-amphiphilic vesicles in biocompatible,aqueous solutions, but it also provides vesicles which are entirelysynthetic, creating an opportunity to tailor the dynamics, structure,rheological and even optical responses of the membrane based on itscomposition. The polymer vesicles of the present invention are called“polymersomes.” Analogous to “liposomes” made from phospholipids, thematerial properties of the polymersome vesicles can be readily measuredusing techniques that have been largely developed for phospholipidvesicles and biological cells. Furthermore, the ability to cross-linkthe polymer building blocks affords a novel opportunity to providemechanical control and stability to the vesicle on the order of thatwhich is provided by the protein skeleton at a cell's plasma membrane.

Polymersomes of the present invention possess membranes capable ofself-repair, adaptability, portability, resilience, and are selectivelypermeable, thereby providing, for example, long-term, reliable andcontrollable vehicles for the delivery or storage of drugs or othercompositions, such as oxygen, to the patient via the bloodstream,gastrointestinal tract, or other tissues, as replacement artificialtissue or soft biomaterial, as optical sensors, and as a structuralbasis for metal or alloy coatings to provide materials having uniqueelectric or magnetic properties for use in high-dielectric or magneticapplications or as microcathodes.

In accordance with the present invention, there are provided vesiclescomprising semi-permeable, thin-walled encapsulating membranes, whereinthe membranes are formed in an aqueous solution, and wherein themembranes comprise one or more synthetic super-amphiphilic molecules.The invention relates to all super-amphiphilic molecules, which havehydrophilic block fractions within the range of 20-50% by weight, andwhich achieve some or all of the above capsular states of matter.Further provided are vesicles and encapsulating membranes, wherein atleast one super-amphiphile molecule is a block copolymer, and whereinthe resulting vesicle is termed a polymersome. The thus providedpolymersomes may be comprised of multi-block copolymers, mostpreferably, but not limited to diblock or triblock copolymers. Moreover,in certain preferred embodiments of the present invention are providedpolymersomes in which all of the super-amphiphile molecules are blockcopolymers. The block copolymers useful in the present invention may beselected from any known block copolymer, including, for examplepolyethylene oxide (PEO), poly(ethylethylene) (PEE), poly(butadiene) (PBor PBD), poly(styrene) (PS), and poly(isoprene) (PI). As needed,monomers for these polymers will be denoted by EO, EE, B or BD, S, andI, respectively.

In addition the present invention provides polymersomes, wherein thevesicles are capable of self-assembly in aqueous solution.

The present invention also provides methods for the preparation ofmixtures of super-amphiphiles from smaller amphiphiles, such asphospholipids up to at least 20% mole fraction, which have also beenshown capable of integrating into stable encapsulating membranes.

Further provided in the present invention are reactive amphiphiles thatcan be covalently cross-linked together, over a many micron-squaredsurface, while maintaining semi-permeability of the membrane.Cross-linked polymersome are characterized as having the ability towithstand exposure to organic solvents, boiling water, dehydration andrehydration in an aqueous solution without visibly or significantlyaffecting the integrity of the membrane.

In addition, the present invention provides polymersomes, wherein thevesicle is biocompatible. Further provided are vesicles for theretention, delivery, and/or extraction of materials, which may requiremembrane biocompatibility and may or may not take advantage of the novelthermal, mechanical, or chemical properties of the surroundingmembranes.

The present invention also provides polymersomes which encapsulate oneor more compositions, such as a drug, therapeutic compound, dye,nutrient, sugar, vitamin, protein or protein fragment, salt,electrolyte, gene or gene fragment, product of genetic engineering,steroid, adjuvant, biosealant, gas, ferrofluid, or liquid crystal. Thepolymersome may be further used to transport an encapsulatable materialto or from its immediately surrounding environment.

Moreover, the present invention provides methods of using thepolymersome or encapsulating membrane to transport one or more of theabove identified compositions to or from a patient in need of suchtransport activity. For example, the polymersome could be used todeliver a drug or therapeutic composition to a patient's tissue or bloodstream, or it could be used to remove a toxic composition from the bloodstream of a patient with, for example, a life threatening hormone orenzyme imbalance.

Also provided by the present invention are methods of preparing apolymersome, wherein the preferred methods of preparation include atleast one step consisting of a film rehydrating step, a bulk rehydratingstep, or an electroforming step.

Further provided are methods for controlling the release of anencapsulated material from a polymersome by modulating and controllingthe composition of the membrane. For example, one preferred method ofcontrolling the release of an encapsulated material from a polymersomeor encapsulating membrane entails cross-linking the membrane. In anotherpreferred method, release of the encapsulated material is controlled byforming the encapsulating membrane from at least one cross-linkableamphiphile and at least one non cross-linkable molecule, followed bysubjecting the thus destabilized membrane to chemical exposure or towaves of propagated light, sound, heat, or motion.

In addition, the present invention provides an encapsulating membranecomprising a semi-permeable, thin-walled encapsulating, amphiphilicmembrane, wherein the membrane is formed around a droplet of oil in amicroemulsion of oil dispersed in an aqueous solution, and wherein themembrane comprises one or more synthetic super-amphiphilic molecules. Ina preferred embodiment, a super-amphiphile layer self-assembles aroundthe oil droplet in water, with or without cross-linking of thesuper-amphiphile.

These and other aspects of the invention will become more apparent fromthe following detailed description when taken in conjunction with thefollowing figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts the molecular assemblies and copolymer structures inwater. FIG. 1A is a schematic representation of diblock copolymerEO₄₀-EE₃₇. The number-average molecular weight is ˜3900 g/mol. For asimple comparison of relative hydrophobic core thickness d, a typicallipid bilayer is schematically shown next to the assembly of copolymers.FIG. 1B depicts aqueous suspensions of EO₄₀-EE₃₇ vesicles in dominantco-existence with rod-like (black arrow) and spherical (gray arrow)micelles. Observations were made by cryo-TEM. The scale bar at lowerleft is 20 nm and the mean lamellar thickness is ˜8 nm with very littlevariation, consistent with unilamellar vesicles.

FIG. 2 depicts giant unilamellar vesicles of EO₄₀-EE₃₇. FIG. 2A depictsa vesicle immediately after electroformation in 100 mM sucrose solution.FIG. 2B depicts encapsulation of 10-kD Texas Red-labeled dextran. FIGS.2C and 2D depict the microdeformation of a polymersome. The arrow marksthe tip of an aspirated projection as it is pulled by negative pressure,ΔP, into the micropipette. As shown, aspiration acts to (i) increasemembrane tension, τ= 1/22ΔPR_(p)/(1- R_(p)/R_(s)), where micropipetteR_(p) and R_(s) are the respective radii of the micropipette and theouter spherical contour; and (ii) expand the original, projected vesiclesurface area, A₀, by the increment ΔA.

FIG. 3 graphically depicts the mechanical properties of polymersomemembranes as assessed by micromanipulation. FIG. 3A shows membraneelasticity in terms of membrane tension versus area expansion. Filledcircles indicate aspiration; open circles indicate graded release. Theupper left inset shows the distribution of measurements for the bendingmodulus, K_(b), as obtained from the initial phase of aspiration. Thelower right inset shows the distribution of measurements for the areaexpansion modulus, K_(a), as obtained from the linear phase ofaspiration. FIG. 3B shows membrane toughness as determined by aspirationto the point of rupture (asterisk). For comparison, aspiration to thepoint of rupture of an electroformed 1-stearoyl-2oleoylphosphatidylcholine (SOPC) lipid vesicle is also shown.

FIG. 4 depicts shape transformations driven by osmotic swelling of asingle polymersome as imaged by phase contrast video microscopy. Thevesicle was formed in 100 mOsm sucrose, and the external sucrosesolution was progressively diluted with distilled water from ˜150 mOsmglucose over a period of 90 min. The transformation is shown as aprogression beginning with FIG. 4A, which shows a giant tubular statethat swells with the initial appearance of interconnected spheres thatconserve vesicle topology, shown in FIGS. 4B through 4C and inset. Thisis followed by the coalescence and disappearance of the small spheres, aform of Ostwald ripening (FIGS. 4D through 4E) before finaltransformation to a single, tensed sphere (FIG. 4F). The entire swellingsequence is predicated on the vesicle's non-zero permeability to wateraccompanied by impermeability to the entrapped sucrose solute.

FIG. 5 indicates thermal and physiological solution stability ofEO₄₀-EE₃₇ vesicles. FIG. 5A shows the membrane's area expansion withincreasing temperature, and its stability at 37° C. The vesicle is heldat a fixed membrane tension of less than 4 mN/m. Relative polymervesicle area, α, is shown against temperature. The overall thermalexpansivity is approximately 1.9×10⁻³ per degree C. FIG. 5B demonstratesthe long-term stability of polymersomes in phosphate buffered saline(PBS).

FIG. 6 shows a Texas Red-phosphatidylethanolamine (PE) lipid probeuniformly integrated into EO₄₀-EE₃₇ vesicles. FIG. 6A shows theuniformity of fluorescence (3 mol %) around an aspirated contour ofmembrane. The radius of the pipette is about 2.5 microns. FIG. 6B showsthat the contour intensity increases linearly up to about 10 mol % TexasRed PE.

FIG. 7 demonstrates the encapsulation of globular proteins. FIG. 7Ashows a 15 μm polymersome encapsulating myoglobin. FIG. 7B shows a 5 μmpolymersome encapsulating hemoglobin. FIGS. 7C and 7D show a 25 μmpolymer vesicle containing fluorescein-tagged bovine serum albumin (BSA)encapsulated at 0.5 g/l 24 hours earlier and viewed in phase contrast(FIG. 7C) and fluorescence (FIG. 7D), respectively.

FIG. 8 depicts a biocompatibility test in which both red cells andpolymersomes were suspended in 250 mOsm phosphate buffered saline in anopened chamber to determine cell adhesion. A polymersome was manipulatedby a micropipette (R_(p)=2 μm) into contact with a granulocyte. Initialcontact at time point 0 is shown in FIG. 8A. FIGS. 8B and 8C depict thecomplete lack of activation of the white cell (which would be observedas extension of pseudopods) or adhesion between the cells at time points62 and 63 seconds, respectively, after initial contact.

FIG. 9 depicts phase contrast images of unilamellar, 15 microns vesiclesof EO₂₆-BD₄₆ with corresponding schematic representations of themembrane before the cross-linking reaction, wherein the osmoticallyinflated vesicles are spherical (FIG. 9A); and after the cross-linkingreaction (FIG. 9C). FIG. 9B depicts a fluid phase vesicle, which hasbeen osmotically deflated, resulting in a flaccid shape, but maintaininga smooth contour. By comparison, FIG. 9D depicts a solid-like,cross-linked membrane, which has been osmotically deflated, resulting ina flaccid shape which is not smooth.

FIG. 10 depicts the stability of an EO₂₆-BD₄₆ vesicle in chloroform.FIG. 10A depicts a vesicle in aqueous solution being pulled into amicropipette (R_(p)=4.5 μm) by negative pressure, ΔP. FIG. 10B depictsthe same vesicle imaged immediately after being placed into chloroform.No noticeable change was observed in the vesicle after 30 minutesexposure to the chloroform (FIG. 10C), nor after return of the vesicleback into the aqueous solution (FIG. 10D).

FIG. 11 depicts the dehydration of a vesicle upon exposure to air. FIG.11A depicts a vesicle in aqueous solution pulled into a micropipette(R_(p)=3.5 μm) by negative pressure, ΔP. FIG. 11B depicts the samevesicle imaged within seconds after its removal from the aqueoussolution and exposure to the air. By comparison, as depicted in FIG.11C, rehydration occurs immediately upon reinsertion of the same vesicleback into the aqueous solution. The original shape is nearly restoredwithin 1 minute, as depicted in FIG. 11D, indicating the retention ofsolutes.

FIG. 12 illustrates the mechanical properties of the cross-linkedpolymersomes. FIG. 12A is the micropipette aspiration curve for asingle, initially flaccid and smooth contour vesicle pulled to a lengthL into a micropipette. R_(p) is the micropipette radius. At highaspiration pressures, the vesicle interior becomes hydrostaticallypressurized. The reversible, initial slope of such a curve is plotted,for a total of ten vesicles, against R_(ves)/R_(p) in FIG. 12B. Thisinitial slope vanishes in the limit of R_(ves)=R_(p), and, above this,resistance to aspiration increases linearly with R_(ves)/R_(p). Theslope of the fitted line provides an estimate of the membrane's elasticshear modulus (μ) which is independent of vesicle size and which is aproperty arising only with cross-linking.

FIG. 13 depicts stabilization of a micro-emulsion by interfacialcross-linking. FIG. 13A depicts amphiphilic PEO-PBD copolymers,self-assembled at the oil-water interface of oil droplets in water(micro-emulsion), with PEO facing the water and PBD facing the oil. Thecross-linking lines represent the new bonds formed due to exposure ofPBD to free radicals. As shown in FIG. 13B, the covalently cross-linkedPBD layer makes the emulsion so stable, that the oil droplets can beaspirated into a micropipette (R_(p)=4 μm) without fragmentation.

FIG. 14 depicts decreased stability of the vesicle fabricated frommixtures of EO₂₆-BD₄₆ and EO₄₀-EE₃₇. FIG. 14A shows 60:40 EO₂₆-BD₄₆:EO₄₀-EE₃₇ vesicle after the cross-linking reaction was completed. FIGS.14B and 14C show the same vesicle aspirated into a micropipette(R_(p)=1.5 μm) by negative pressure, ΔP=2 cm of water, and ΔP=10 cm ofwater, respectively. The increased pressure in FIG. 14C leads toperforation of the membrane and leakage of its contents.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides stable vesicles comprising,semi-permeable, thin-walled encapsulating membranes, tens of nanometersto tens of microns in diameter, made by self-assembly in various aqueoussolutions of purely synthetic, amphiphilic molecules having an averagemolecular weight of many kilograms per mole. Such molecules are referredto as “super-amphiphiles” because of their large molecular weight incomparison to other amphiphiles, such as the phospholipids andcholesterol of eukaryotic cell membranes.

The relevant class of super-amphiphilic molecules is represented by, butnot limited to, block copolymers, e.g., hydrophilic polyethyleneoxide(EO) linked to hydrophobic polyethylethylene (EE). The syntheticdiversity of block copolymers provides the opportunity to make a widevariety of vesicles with material properties that greatly expand what iscurrently available from the spectrum of naturally occurringphospholipids. For the purposes of this invention, although technicallydistinct and distinguished on the basis of molecular weight, the terms“super-amphiphile” and “amphiphile” are used interchangeably, forexample, to refer to the block copolymers of the present invention.

In a preferred embodiment, the invention further provides for thepreparation of vesicles harboring mixtures of super-amphiphiles andsmaller amphiphiles, such as phospholipids up to at least 20% molefraction. The latter have been shown to be capable of integrating intostable vesicles of super-amphiphiles.

“Vesicles,” as the term is used in the present invention, areessentially semi-permeable bags of aqueous solution as surrounded(without edges) by a self-assembled, stable membrane composedpredominantly, by mass, of either amphiphiles or super-amphiphiles whichself-assemble in water or aqueous solution. Thus, a biological cellwould, in general, represent a naturally occurring vesicle. Smallervesicles are also found within biological cells, and many of thestructures within a cell are vesicular. The membrane of an internalvesicle serves the same purpose as the plasma membrane, i.e., tomaintain a difference in composition and an osmotic balance between theinterior of the vesicle and the exterior. Many additional functions ofcell membranes, such as in providing a two-dimensional scaffold forenergy conversion can be added to compartmentalization roles. For anintracellular vesicle, the environment outside the vesicle is thecytoplasm.

The “cell membrane” or “plasma membrane” is a complex, contiguous,self-assembled, complex fluid structure comprised of amphiphilic lipidsin a bilayer with associated proteins and which defines the boundary ofevery cell. It is also referred to as a “biomembrane.” “Phospholipids”comprise lipid substances, which occur in cellular membranes and containesters of phosphoric acid, such as sphingomyelins, and includephosphatides, phospholipins and phospholipoids.

Synthetic amphiphiles having molecular weights less than a fewkilodaltons, like natural amphiphiles, are pervasive as self-assembled,encapsulating membranes in water-based systems. These include complexfluids, soaps, lubricants, microemulsions consisting of oil droplets inwater, as well as biomedical devices such as vesicles. An “encapsulatingmembrane,” as the term is used in the present invention, is a vesicle inall respects except for the necessity of aqueous solution. Encapsulatingmembranes, by definition, compartmentalize by being semi- or selectivelypermeable to solutes, either contained inside or maintained outside ofthe spatial volume delimited by the membrane. Thus, a vesicle is acapsule in aqueous solution, which also contains aqueous solution.However, the interior or exterior of the capsule could also be anotherfluid, such as an oil or a gas. A “capsule,” as the term is used in thepresent invention, is the encapsulating membrane plus the space enclosedwithin the membrane.

“Complex fluids” are fluids that are made from molecules that interactand self-associate, conferring novel Theological, optical, or mechanicalproperties on the fluid itself. Complex fluids are found throughoutbiological and chemical systems, and include materials such asbiological membranes or biomembranes, polymer melts and blends, andliquid crystals. The self-association and ordering of the moleculeswithin the fluid depends on the interaction between component parts ofthe molecules, relative to their interaction with solvent, if present.

The plasma membrane is a “lipid bilayer” comprising a double layer ofphospholipid/diacyl chains, wherein the hydrophobic fatty acid tails ofthe phospholipids face each other and the hydrophilic polar heads ofeach layer face outward toward the aqueous solutions (see FIG. 1A).Numerous receptors, steroids, transporters and the like are embeddedwithin the bilayer of a typical cell. Thus, a “lipid vesicle” or“liposome,” is a vesicle surrounded by a membrane comprising one or morephospholipids. Throughout the specification the terms “cell membrane,”“plasma membrane,” “lipid membrane,” and “biomembrane” may be usedinterchangeably to refer to the same lipid bilayer surrounding a cell orvesicle.

A “membrane”, as the term is used in this invention, is a spatiallydistinct collection of molecules that defines a 2-dimensional surface in3-dimensional space, and thus separates one space from another in atleast a local sense. Such a membrane must also be semi-permeable tosolutes. It must also be sub-microscopic (less than optical wavelengthsof around 500 nm) in its thickness (d in FIG. 1A), as resulting from aprocess of self-assembly. It can have fluid or solid properties,depending on temperature and on the chemistry of the amphiphiles fromwhich it is formed. At some temperatures, the membrane can be fluid(having a measurable viscosity), or it can be solid-like, with anelasticity and bending rigidity. The membrane can store energy throughits mechanical deformation, or it can store electrical energy bymaintaining a transmembrane potential. Under some conditions, membranescan adhere to each other and coalesce (fuse). Soluble amphiphiles canbind to, and intercalate within a membrane.

A “bilayer membrane” (or simply “bilayer(s)”) for the purposes of thisinvention is a self assembled membrane of amphiphiles orsuper-amphiphiles in aqueous solutions.

“Polymersomes” are vesicles, which are assembled from synthetic polymersin aqueous solutions. Unlike liposomes, a polymersome does not includelipids or phospholipids as its majority component. Consequently,polymersomes can be thermally, mechanically, and chemically distinctand, in particular, more durable and resilient than the most stable oflipid vesicles. The polymersomes assemble during processes of lamellarswelling, e.g., by film or bulk rehydration or through an additionalphoresis step, as described below, or by other known methods. Likeliposomes, polymersomes form by “self assembly,” a spontaneous,entropy-driven process of preparing a closed semi-permeable membrane.

Because of the bilayer's perselectivity, materials may be “encapsulated”in the aqueous interior or intercalated into the hydrophobic membranecore of the polymersome vesicle of the present invention. Numeroustechnologies can be developed from such vesicles, owing to the numerousunique features of the bilayer membrane and the broad availability ofsuper-amphiphiles, such as block copolymers.

The synthetic polymersome membrane can exchange material with the“bulk,” i.e., the solution surrounding the vesicles. Each component inthe bulk has a partition coefficient, meaning it has a certainprobability of staying in the bulk, as well as a probability ofremaining in the membrane. Conditions can be predetermined so that thepartition coefficient of a selected type of molecule will be much higherwithin a vesicle's membrane, thereby permitting the polymersome todecrease the concentration of a molecule, such as cholesterol, in thebulk. In a preferred embodiment, phospholipid molecules have been shownto incorporate within polymersome membranes by the simple addition ofthe phospholipid molecules to the bulk. In the alternative, polymersomescan be formed with a selected molecule, such as a hormone, incorporatedwithin the membrane, so that by controlling the partition coefficient,the molecule will be released into the bulk when the polymersome arrivesat a destination having a higher partition coefficient.

The polymersomes of the present invention are formed from synthetic,amphiphilic copolymers. An “amphiphilic” substance is one containingboth polar (water-soluble) and hydrophobic (water-insoluble) groups.“Polymers” are macromolecules comprising connected monomeric units. Themonomeric units may be of a single type (homogeneous), or a variety oftypes (heterogeneous). The physical behavior of the polymer is dictatedby several features, including the total molecular weight, thecomposition of the polymer (e.g., the relative concentrations ofdifferent monomers), the chemical identity of each monomeric unit andits interaction with a solvent, and the architecture of the polymer(whether it is single chain or branched chains). For example, inpolyethylene glycol (PEG), which is a polymer of ethylene oxide (EO),the chain lengths which, when covalently attached to a phospholipid,optimize the circulation life of a liposome, is known to be in theapproximate range of 34-114 covalently linked monomers (EO₃₄ to EO₁₁₄).

The preferred class of polymer selected to prepare the polymersomes ofthe present invention is the “block copolymer.” Block copolymers arepolymers having at least two, tandem, interconnected regions ofdiffering chemistry. Each region comprises a repeating sequence ofmonomers. Thus, a “diblock copolymer” comprises two such connectedregions (A-B); a “triblock copolymer,” three (A-B-C), etc. Each regionmay have its own chemical identity and preferences for solvent. Thus, anenormous spectrum of block chemistries is theoretically possible,limited only by the acumen of the synthetic chemist.

In the “melt” (pure polymer), a diblock copolymer may form complexstructures as dictated by the interaction between the chemicalidentities in each segment and the molecular weight. The interactionbetween chemical groups in each block is given by the mixing parameteror Flory interaction parameter, χ, which provides a measure of theenergetic cost of placing a monomer of A next to a monomer of B.Generally, the segregation of polymers into different ordered structuresin the melt is controlled by the magnitude of χ N, where N isproportional to molecular weight. For example, the tendency to formlamellar phases with block copolymers in the melt increases as χ Nincreases above a threshold value of approximately 10.

A linear diblock copolymer of the form A-B can form a variety ofdifferent structures. In either pure solution (the melt) or diluted intoa solvent, the relative preferences of the A and B blocks for eachother, as well as the solvent (if present) will dictate the ordering ofthe polymer material. In the melt, numerous structural phases have beenseen for simple AB diblock copolymers.

To form a stable membrane in water, the absolute minimum requisitemolecular weight for an amphiphile must exceed that of methanol HOCH₃,which is undoubtedly the smallest canonical amphiphile, with one endpolar (HO—) and the other end hydrophobic (—CH₃). Formation of a stablelamellar phase more precisely requires an amphiphile with a hydrophilicgroup whose projected area, when viewed along the membrane's normal, isapproximately equal to the volume divided by the maximum dimension ofthe hydrophobic portion of the amphiphile (Israelachvili, inIntermolecular and Surface Forces, 2^(nd) ed., Pt3 (Academic Press, NewYork) 1995).

The most common lamellae-forming amphiphiles also have a hydrophilicvolume fraction between 20 and 50%. Such molecules form, in aqueoussolutions, bilayer membranes with hydrophobic cores never more than afew nanometers in thickness. The present invention relates to allsuper-amphiphilic molecules which have hydrophilic block fractionswithin the range of 20-50% by volume and which can achieve a capsularstate. The ability of amphiphilic and super-amphiphilic molecules toself-assemble can be largely assessed, without undue experimentation, bysuspending the synthetic super-amphiphile in aqueous solution andlooking for lamellar and vesicular structures as judged by simpleobservation under any basic optical microscope or through the scatteringof light.

For typical phospholipids with two acyl chains, temperature can affectthe stability of the thin lamellar structures, in part, by determiningthe volume of the hydrophobic portion. In addition, the strength of thehydrophobic interaction, which drives self-assembly and is required tomaintain membrane stability, is generally recognized as rapidlydecreasing for temperatures above approximately 50° C. Such vesiclesgenerally are not able to retain their contents for any significantlength of time under conditions of boiling water.

Upper limits on the molecular weight of synthetic amphiphiles which formsingle component, encapsulating membranes clearly exceed the manykilodalton range, as concluded from the work of Discher et al., (1999),which contributes foundationally to the present invention, and is hereinincorporated by reference.

Block copolymers with molecular weights ranging from about 2 to 10kilograms per mole can be synthesized and made into vesicles when thehydrophobic volume fraction is between about 20% and 50%. Diblockscontaining polybutadiene are prepared, for example, from thepolymerization of butadiene in cyclohexane at 40° C. usingsec-butyllithium as the initiator. Microstructure can be adjustedthrough the use of various polar modifiers. For example, purecyclohexane yields 93% 1,4 and 7% 1,2 addition, while the addition ofTHF (50 parts per Li) leads to 90% 1,2 repeat units. The reaction may beterminated with, for example, ethyleneoxide, which does not propagatewith a lithium counterion and HCl, leading to a monofunctional alcohol.This PB-OH intermediate, when hydrogenated over a palladium (Pd) supportcatalyst, produces PEE-OH. Reduction of this species with potassiumnaphthalide, followed by the subsequent addition of a measured quantityof ethylene oxide, results in the PEO-PEE diblock copolymer. Manyvariations on this method, as well as alternative methods of synthesisof diblock copolymers are known in the art; however, this particularpreferred method is provided by example, and one of ordinary skill inthe art would be able to prepare any selected diblock copolymer.

For example, if PB-PEO diblock copolymers were selected, the synthesisof PB-PEO differs from the previous scheme by a single step, as would beunderstood by the practitioner. The step by which PB-OH is hydrogenatedover palladium to form PEO-OH is omitted. Instead, the PB-OHintermediate is prepared, then it is reduced, for example, usingpotassium naphthalide, and converted to PB-PEO by the subsequentaddition of ethylene oxide.

In yet another example, triblock copolymers having a PEO end group canalso form polymersomes using similar techniques. Various combinationsare possible comprising, e.g., polyethylene, polyethylethylene,polystyrene, polybutadiene, and the like. For example, a polystyrene(PS)-PB-PEO polymer can be prepared by the sequential addition ofstyrene and butadiene in cyclohexane with hydroxyl functionalization,re-initiation and polymerization. PB-PEE-PEO results from the two-steppolymerization of butadiene, first in cyclohexane, then in the presenceof THF, hydrolyl functionalization, selective catalytic hydrogenation ofthe 1,2PB units, and the addition of the PEO block.

A plethora of molecular variables can be altered with these illustrativepolymers, hence a wide variety of material properties are available forthe preparation of the polymersomes. ABC triblocks can range frommolecular weights of 3,000 to at least 30,000 g/mol. Hydrophiliccompositions should range from 20-50% in volume fraction, which willfavor vesicle formation. The molecular weights must be high enough toensure hydrophobic block segregation to the membrane core. The Floryinteraction parameter between water and the chosen hydrophobic blockshould be high enough to ensure said segregation. Symmetry can rangefrom symmetric ABC triblock copolymers (where A and C are of the samemolecular weight) to highly asymmetric triblock copolymers (where, forexample, the C block is small, and the A and B blocks are of equallength).

TABLE 1 lists some of the synthetic super-amphiphiles of many kilogramsper mole in molecular weight, which are capable of self-assembling intosemi-permeable vesicles in aqueous solution. The panel of preferredPEO-PEE block copolymers ranges in molecular weight from 1400 to 8700,with hydrophilic volume fraction, f_(EO), ranging from 20% to 50%. Thepolydispersity indices for the resulting polymers do not exceed 1.2,confirming a narrow polydispersity. TABLE 1 Molecular WeightSuper-Amphiphile * (g/mol) ** Vol. fraction EO (±1%) ^(‡) EO₄₀-EE₃₇ 390039% EO₄₃-EE₃₅ 3900 42% EO₄₉-EE₃₇ 4300 44% EO₂₆-PB₄₆ 3600 28% EO₃₁-PB₄₆3800 31% EO₄₂-PB₄₆ 5300 37% EO₃₃-S₁₀-I₂₂ ^(‡‡) 3900 33% EO₄₈-EE₇₅-EO₄₈8400 44%* EO = ethyleneoxide, EE = ethylethylene, B = butadiene, S = styrene, I= isoprene** Molecular Weight denotes number-average molecular weight (Mn) ± 50g/mol^(‡)Volume fractions determined by NMR.^(‡‡)EO-S-I has number-average molecular weight for the respectiveblocks of 1440, 1008, 1470 g/mol.

TABLE 1 is intended only to be representative of the syntheticsuper-amphiphiles suitable for use in the present invention. It is notintended to be limiting. The table can be effectively used to selectwhich block copolymers will form lamellar phases and vesicles. One ofordinary skill in the art will readily recognize many other suitableblock copolymers that can be used in the preparation of polymersomesbased on the teachings of the present invention.

In a preferred embodiment of the present invention, polymersomescomprise the selected polymer polyethyleneoxide-polyethylethylene(EO₄₀-EE₃₇), also designated OE-7, and having a chain structuret-butyl-[CH₂—CH(C₂H₅)]₃₇—[CH₂—CH₂—O]₄₀—H. The molecule's averagemolecular weight is about five to ten times greater than that of typicalphospholipids in natural membranes. The resulting polymersome membraneis found to be at least 10 times less permeable to water than commonphospholipid bilayers.

A vesicle suspended in water which encapsulates impermeable solutes andwhich has a non-zero membrane permeability to water can be osmoticallyforced to change its shape. Shape transformations of vesicle capsules,the simple red blood cell included, have generally been correlated withenergy costs or constraints imposed by vesicle area, the number ofmembrane molecules making up the vesicle area, the volume enclosed bythe vesicle, and the curvature elasticity of the membrane (see, e.g.,Deuling et al., J. Phys. 37:1335 (1976); Svetina et al., Eur. Biophys.17:101 (1989); Seifert et al., Phys. Rev. A 44:1182 (1991)). Theoreticaland experimental efforts on fluid lipid bilayers (e.g., Seifert andLipowsky, in Handbook of Biological Physics, chap. 8; Dobereiner et al.,Phys. Rev. E 55:4458 (1997)) have separated the elasticity in bendingbetween a local, K_(b)-scaled curvature energy term that includes aspontaneous curvature, c_(o), and a more non-local,area-difference-elasticity term predicated on monolayer unconnectednessin spherical-topology vesicles. To oppose any relaxation of leaflet areadifference, a lack of lipid transfer or “flip-flop” between layers mustbe postulated. Only with such a non-local area difference term can avesicle maintain in apparent equilibrium the type of multi-sphere andbudded morphologies observable in both lipid systems (Chaieb et al.,Phys. Rev. E 58:7733 (1998)) and in the osmotically deflatedpolymersomes shown in FIG. 4. Because worm-like and spherical micellesare also in evidence (FIG. 1B), however, a non-zero c_(o) also appearslikely. Heterogeneity in the morphology of polymersomes, both small(FIG. 1B) and large vesicles (FIG. 4), denotes, however, an importantcontribution from monolayer area difference, a process-dependent featurethat arises upon vesicle closure.

The tool that has been used to measure many of the material propertiesof bilayer vesicles is “micropipette aspiration” as applied in FIG. 2.In micropipette aspiration, the rheology and material properties ofmicron-sized objects are measured using glass pipettes. Small, microndiameter pipettes are used to pick up, deform and manipulatemicron-sized objects, such as giant lipid vesicles. The aspirationpressure is controlled by manometers, in which the hydrostatic pressurein a reservoir connected to the micropipette is varied in relation to afixed reference. Pressure may be varied with a resolution of microns ofH₂O (or 10⁻⁶ atm).

A deformable object is aspirated using a pressure driving force (orsuction pressure), ΔP, and the object is drawn within the pipette to aprojection length L_(P). For a liquid, the tension in the membrane, T,can be obtained from the Law of Laplace in terms of the pressure drivingforce, the pipette inner radius, R_(P), the vesicular outer diameter,R_(S), and the length of the projection. This technique has been used tomeasure the moduli of deformation and strength of lipid vesiclemembranes, such as the bending modulus (K_(b)), the area expansionmodulus (K_(a)), the critical areal strain to the point of failure(α_(c)) and the toughness (E_(c) or T_(f)) (the energy stored in thevesicle prior to failure) (see, e.g., Evans et al., J. Phys. Chem.91:4219 (1987); Needham et al., Biophys. J. 58:997 (1990)). The bendingmodulus is measured by exerting small tensions on the membrane, tosmooth out thermally-driven surface undulations. At larger tensions,beyond a crossover tension at which the undulations of the membrane havebeen smoothed, the tension acts to stretch the membrane in-plane againstthe cohesive hydrophobic forces holding the membrane together. The areaexpansion modulus is the unit tension required for a unit increase instrain. The critical area strain is obtained by stressing the membraneto the point of cohesive failure. Thus, micropipette aspiration is apowerful tool for exploring the interfacial and material properties ofthe polymersomes of the present invention.

TABLE 2 demonstrates that the membrane mechanical properties of severalpreferred polymer vesicles are independent of the different methods ofassembly in aqueous media. K_(a) falls within the broad range of lipidmembrane measurements. In contrast, the giant polymersomes of thepresent invention prove to be almost an order of magnitude tougher andsustain far greater areal strain under tension before rupture than anynaturally occurring or synthetic vesicle known in the art. Membranesformed from the preferred super-amphiphilic diblocks of eitherpolyethyleneoxide-polyethylethylene or polyethyleneoxide-polybutadienehave also been shown to be thicker than lipid membranes, providing aphysical basis for understanding the enhanced toughness, as well as thereduced permeability. TABLE 2 Method of Super-Amphiphile Formation Ka(mN/m) * α_(c) = (ΔA/Ao) ** d: thickness *** EO₄₀-EE₃₇ Film 115 ± 270.20 ± 0.07 8 ± 1 nm Rehydration [20 vesicles]  [5 vesicles] Electro-120 ± 20 0.19 ± 0.02 formation [21 vesicles]  [6 vesicles] EO₂₆-B₄₆ Film 80 ± 34 9 ± 1 nm Rehydration [5 vesicles] Bulk  94 ± 10 Rehydration [4vesicles] EO₅₀-B₅₄ Film  82 ± 23 0.30 Rehydration [9 vesicles] [2vesicles]* K_(a) is the elastic modulus for area expansion.** α_(c) is the critical area strain at which an initially unstressedmembrane will rupture.*** The hydrophobic core thickness, d, is determined by electronmicroscopy.

Preferred assemblies of the present invention can withstandexceptionally severe environmental conditions of temperature andexposure to solvent. TABLE 3 indicates the result of suspending vesiclesof EO₄₀-EE₃₇ in a sterilizing aqueous solution of ethanol in phosphatebuffered saline (PBS) for at least 15 minutes. Many phospholipidvesicles would be unstable under such solvent conditions. TABLE 3 25%EtOH in PBS PBS Vesicle per ml* 7.2 × 10⁴ 9.0 × 10⁴ Vesicle diameter(μm) 9.7 ± 5.4 8.6 ± 4.1*5 μl of vesicles in 247 mOsm sucrose were added to 200 μl of 25%EtOH/PBS or PBS.

The methods and examples that follow make use of and extend the abovecharacterization methods and concepts.

A. Preparation of Polymersomes

In the preferred embodiments of the present invention, the polymersomesare comprised of a subset class of block copolymers—the “amphiphilicblock copolymers,” meaning that in a diblock copolymer, region A ishydrophilic and region B is hydrophobic. Like phospholipid amphiphiles,block copolymer amphiphiles self-assemble into lamellar phases atcertain compositions and temperatures and can form closed bilayerstructures capable of encapsulating aqueous materials. Vesicles fromblock-copolymer amphiphiles have the additional advantage of being madefrom synthetic molecules, permitting one of ordinary skill to applyknown synthetic methods to greatly expand the types of vesicles and thematerial properties that are possible based upon the presently disclosedand exemplified applications.

The diblock copolymers used to form the super-amphiphile vesicles of theinvention may be synthesized by any method known to one of ordinaryskill in the art for synthesizing copolymers. Such methods are taught,for example, by Hajduk et al., 1998; Hillmyer and Bates, Macromolecules29, 6994 (1996); and Hillmyer et al., Science 271:976 (1996)), althoughthe practitioner need not be so limited. Nevertheless, use of the Batesmethod results in very low polydispersity indices for the synthesizedpolymer (not exceeding 1.2), and make the methods particularly suitedfor use in the present invention, at least from the standpoint ofhomogeneity. Indeed, the demonstrated ability to make stable vesiclesfrom PEO-PEE with up to at least 20% mole fraction of phospholipidstrongly indicates that polydispersity need not be limiting in theformation of stable vesicles.

Vesicles can be prepared by any method known to one of ordinary skill inthe art. However, the preferred method of preparation is filmrehydration, which has yielded vesicles for all copolymers that havebeen found to be capable of forming vesicles. Other methods can be usedas described below, but they do not guarantee vesicle formation for all“vesicle-forming” amphiphiles.

(1) Film Rehydration

In the film rehydration method, in general, pure amphiphiles aredissolved in any suitable solvent that can be completely evaporatedwithout distracting the amphiphile, at concentrations preferably rangingfrom 0.1 to 50 mg/ml, more preferably from 1 to 10 mg/ml, mostpreferably yielding 1 μmol/ml solution. The preferred solvent for thispurpose in the present invention is chloroform. When amphiphile mixturesare used, each component of the mixture must be dissolved separately andmixed in a measured aliquot of the solvent to obtain a solutioncomprising the desired ratio of components. The resulting solution isplaced into a glass vial, and the solvent is evaporated to yield a thinfilm, having a preferable density of approximately 0.01 μmol/cm².

When chloroform is used as the solvent, the solution is evaporated undernitrogen gas and under applied vacuum for three hours or longer, untilevaporation is completed. After complete evaporation of the solvent, anaqueous solution comprising the “to be encapsulated” material is addedto the glass vial, yielding a preferred 0.1% (w/w) solution. Vesiclesform spontaneously at room temperature in a time dependent mannerranging from several hours to several days, depending on the selectedamphiphile and the aqueous solvent and the ratio between them.Temperature may be used as a control variable in this process offormation. The yield of vesicles can be optimized without undueexperimentation by the selection of aqueous components and by tuning theexperimental conditions, such as concentration and temperature.

(2) Bulk Rehydration

In the alternative, the pure amphiphile can be mixed with an aqueoussolution to a preferred concentration of 0.01-1% (w/w), most preferably0.1% (w/w), then dissolved into small aggregates (with dimensions ofseveral microns) by mixing. When the aggregates are then incubatedwithout any perturbance for several hours to several days, depending onthe amphiphile, aqueous solvent and temperature, vesicles formspontaneously on the aggregate surface, from which they can bedissociated by gentle mixing or shaking.

(3) Electroformation

Polymersomes are more preferably made by electroformation, by using theadapted methods of Angelova et al., Prog. Coll. Polym. Sci. 89:127(1992), which have been previously used by Hammer as reported by Longoet al., Biophys. J. 73:1430 (1997) (both are herein incorporated byreference), although the preparation need not be so limited. Briefly, byexample, 20 μl of the amphiphile solution (in chloroform or othersolvent made to preferable concentration 1 μmol/ml) is deposited as afilm on two 1 mm-diameter adjacent platinum wire electrodes held in aTeflon frame (5 mm separation of the electrodes). The solvent is thenevaporated under nitrogen, followed by vacuum drying for 3 to 48 hours.The Teflon frame and coated electrodes are then assembled into achamber, which is then sealed with coverslips. Preferably, thetemperature and humidity of the chamber are controlled. The chamber issubsequently filled with a degassed aqueous solution, e.g., glucose orsucrose, preferably about 0.1 to 0.25 M or with a protein solutioncontaining, for example, a globin.

To begin generating polymersomes from the film, an alternating electricfield is applied to the electrodes (e.g., 10 Hz, 10 V) while the chamberis mounted and viewed on the stage of an inverted microscope. Giantvesicles attached to the film-coated electrode are visible after 1 to 60min. The vesicles can be dissociated from the electrodes by lowering thefrequency to about 3 to 5 Hz for at least 15 min, and by removing thesolution from the chamber into a syringe.

In spite of several techniques used, it was found in practicing thepresent invention, that for each of the particular amphiphiles studied,the method selected for vesicle formation did not alter the mechanicalproperties of the resulting vesicles (TABLE 2).

(4) Fragmentation

The size of giant polymersome can be decreased to any average vesiclesize as desired for a given application by filtration throughpolycarbonate filter (Osmonics, Livermore, Calif.). As an example,5.5±3.0 μm vesicles were filtered through a 1.0 micron polycarbonatefilter. The size of the vesicles decreased to 2.4±0.36 μm.

B. Characterization of Polymersomes

The structure of an exemplified polymersome vesicle can be characterizedby the following generalized method. In a preferred embodiment, 1% (w/w)of the amphiphile is solubilized in aqueous solution, and the vesiclesself-assemble during the solubilization process. Thin films (ca. 100 nm)of the vesicular solution suspended within the pores of amicroperforated grid are prepared in an isolated chamber with controlledtemperature and humidity (Lin et al., Langmuir, 8:2200 1992). The sampleassembly is then rapidly vitrified with liquid ethane at its meltingtemperature (˜90 K), and then kept under liquid nitrogen until loadedonto a cryogenic sample holder (Gatan 626) (Lin et al., (1992)).

The morphologies of the polymersomes may be visualized bycryo-transmission electron microscopy (cryo-TEM or CTEM), bytransmission electron microscopy (TEM), such as on a Phillips EM410transmission electron microscope operating at an acceleration voltage of80-100 kV, by inverted stage microscopy, or by any other means known inthe art for visualizing vesicles. Cryo-TEM images revealed, at 1 nmresolution, the mean lamellar thickness of the hydrophobic core, whichwas ˜8 to 9 nm for both the EO₄₀-EE₃₇ and EO₂₆-PD₄₆ membranes as listedin TABLE 2.

Small angle X-ray and neutron scattering (SAXS and SANS) analyses arewell suited for quantifying the thickness of the membrane core (Won etal., 1999) or any internal structure. SAXS and SANS can provide precisecharacterization of the membrane dimensions, including theconformational characteristics of the PEO corona that stabilizes thepolymersome in an aqueous solution. Neutron contrast is created bydispersing the vesicles in mixtures of H₂O and D₂O, thereby exposing theconcentration of water as a function of distance from the hydrophobiccore.

Size distribution can be determined directly by microscopic observation(light and/or electron microscopy), by dynamic light scattering, or byother known methods. Polymersome vesicles can range in size from tens ofnanometers to hundreds of microns in diameter. According to acceptedterminology developed for lipid vesicles, small vesicles can be as smallas about 1 nm in diameter to over 100 nm in diameter, although theytypically have diameters in the tens of nanometers. Large vesicles rangefrom 100 to 500 nm in diameter. Both small and large vesicles are bestperceived as such by light scattering and electron microscopy. Giantvesicles are generally greater than 0.5 to 1 μm in diameter, and cangenerally be perceived as vesicles by optical microscopy.

Small vesicles can be as small as 1 nm in diameter to over 100 nm indiameter, although they typically have diameters in the tens ofnanometers. Large vesicles range from 100 to 1000 nm in diameter,preferably from 500 to 1000 nm. Giant vesicles are generally greaterthan 1 μm in diameter. The preferred polymersome vesicles range of 20 nmto 100 μm, preferably from 1 μm to 75 μm, and more preferably from 1 μmto 50 μm.

The disclosed methods of preparation of the polymersomes areparticularly preferred because the vesicles are prepared without the useof co-solvent. Any organic solvent used during the disclosed synthesisor film fabrication method has been completely removed before the actualvesicle formation. Therefore, the polymersomes of the present inventionare free of organic solvents, distinguishing the vesicles from those ofthe prior art and making them uniquely suited for bio-applications.

The methods of analysis applied in a preferred embodiment of theinvention provide a clear basis for applications of mass retention,delivery, and extraction, which may require membrane biocompatibility,and which may or may not take advantage of the novel thermal,mechanical, or chemical properties of the membranes. By “biocompatible”is meant a substance or composition which can be introduced into ananimal, particularly into a human, without significant adverse effect.For example, when a material, substance or composition of matter isbrought into a contact with a viable white blood cell, if the material,substance or composition of matter is toxic, reactive or biologicallyincompatible, the cells will perceive the material as foreign, harmfulor immunogenic, causing activation of the immune response, and resultingin immediate, visible morphological changes in the cell. A “significant”adverse effect would be one which is considered sufficiently deleteriousas to preclude introducing a substance into the patient.

To confirm one level of biocompatibility of the polymersomes,preliminary evaluations were performed by bringing the polymersomes intocontact with white blood cells, such as granulocytes, as seen in FIG.8A. Even after prolonged contact (over one minute) with thepolymersomes, the white cells remained intact and unchanged (FIGS. 8Band 8C). No adhesion was observed, and the polymersomes caused noactivation of the white blood cells, thus demonstrating thebiocompatibility of the polymersomes.

If there were adhesion between vesicles and blood cells, micropipetteaspiration could also be used to measure the inter-lamellar adhesionenergy. If two vesicles or a cell and vesicle are manipulated intocontact and adherent, then the inter-lamellar adhesion energy density yis determined from Young's equation, γ=τ(1−cos θ), where θ is themeasurable contact angle between the two surfaces, τ is the tensionrequired to peel the membranes apart. In the case of adhesion beingstrong enough to induce membrane cohesion, aspiration can again be usedto directly observe the resulting coalescence of two vesicles (fusion),as well as the adsorption and intercalation of soluble objects (such as,surfactants or micelles) into the membrane.

C. Encapsulation into Polymersomes

An enormously wide range of materials can be encapsulated within apolymersome vesicle. In fact, to date no molecule has been found thatcannot be encapsulated. Among the exemplary molecules that have beenencapsulated are: proteins and proteinaceous compositions, e.g.,myoglobin, hemoglobin and albumin, sugars and other representativecarriers for drugs, therapeutics and other biomaterials, e.g., 10 kDadextran, sucrose, and phosphate buffered saline, as well as markerpreparations. Encapsulation applications range, without limitation from,e.g., drug delivery (aqueously soluble drugs), to optical detectors(fluorescent dyes), to the storage of oxygen (hemoglobin).

A variety of fluorescent dyes of the type that can be incorporatedwithin the polymersomes could include small molecular weightfluorophores, such as FITC, and fluorophores attached to dextrans of aladdered sequence of molecular weights. Imaging of the fluorescent corecan be accomplished by standard fluorescent videomicroscopy.Permeability of the polymersome to the fluorophore can be measured bymanipulating the fluorescently-filled vesicle with aspiration, andmonitoring the retention of fluorescence against a measure of time.

Phosphate buffered saline (PBS; 10 mM phosphate buffer, 2.7 mM KCl, and137 M NaCl) and other electrolytes, such as, but not limited to, KF orKI can be added during the vesicle preparation and be easilyencapsulated by rehydration. The electroformation method is not veryefficient in the presence of electrolytes.

FIG. 7 demonstrates the encapsulation of globular proteins by filmrehydration. As shown, EO₄₀-EE₃₇ vesicles were electroformed with 10 g/Lmyoglobin dissolved in 289 mOsm sucrose solution (FIG. 7A), and with 10g/L hemoglobin dissolved in 280 mOsm PBS/sucrose solution (FIG. 7B).FIGS. 7C and 7D show a polymer vesicle containing fluorescein-taggedbovine serum albumin (BSA) encapsulated at 0.5 g/l.

D. Cross-Linking of the Polymersomes

In a preferred embodiment, the invention provides reactive amphiphilesthat can be covalently cross-linked together, over a many micron-squaredsurface, while maintaining the semi-permeability of the membrane.Cross-linked polymersomes are particularly useful in applicationsrequiring stability of the vesicle membranes and durable retention ofthe encapsulated materials. By cross-linked is meant covalentlyinterconnected; i.e., completely cross-linked vesicle have all themembrane components covalently interconnected into a giant singlemolecule; cross-linked vesicles have interconnected componentsthroughout their entire surface; and partly cross-linked vesiclescontain patches of the interconnected components.

Cross-linking of the amphiphiles can be achieved using doublebond-containing blocks, such as polybutadiene, which can be readilycoupled by standard cross-linking reactions. In a preferred embodimentof the present invention, the vesicles are cross-linked by free radicalsgenerated with combination of an initiator, such as K₂S₂O₈, and a redoxcoupler, such as Na₂S₂O₅/FeSO₄.7H₂O (Won et al., 1999). Although anysuitable pairing of an initiator and a redox coupler may be selected byone of ordinary skill in the art to cause the cross-linking reaction,the suggested compounds have been found to be particularly suited toeffect the cross-linking of the exemplified amphiphiles of the presentinvention. In the preferred and exemplified embodiment, the osmolarityof the cross-linking reagents is adjusted to match the osmolarity of theencapsulated material, and the components are mixed in the followingorder and volume ratios relative to sample:K₂S₂O₈:Na₂S₂O₅:FeSO₄=1:0.5:0.02. Due to instabilities of the sulfates,K₂S₂O₈ and Na₂S₂O₅ must be prepared within a few days of performing thereaction and FeSO₄ within several minutes of its use to ensure efficientcross-linking of the amphiphiles.

Of course, the cross-linking mechanism need not to be limited to redoxreaction methods, such as the one disclosed above. Cross-linking can becarried out by a variety of alternative and known techniques, includingbut not limited to, ⁶⁰Co γ-irradiation (Hentze et al., Macromolecules32: 5803-5809 (1999)), or by visible or UV light irradiation with anincorporated sensitizer, such as 3,3,3′,3′-tetramethyldiocta-decylindocarbocyanine (DiI(C₁₈)). (DiI(C₁₈) is an amphiphilic sensitizing dyewhich can generate oxygen free radicals when irradiated with green or UVlight (Mueller et al., Polymer Preprints (ACS) 40(2):205 (1999)). It hasalready been established that this particular dye, as well as otherdyes, can be incorporated into the polymersome membrane during vesiclepreparation, or even after vesicle formation, in relatively largeamounts as observed by fluorescent microscopy.

E. Permeability of the Polymersome Membrane, and Transport ofEncapsulated Material

(1) Water Permeability

Polymersomes, as exemplified by EO₄₀-EE₃₇, can be substantially lesspermeable to water than phospholipid membranes, which suggests manybeneficial applications for the polymersomes. To measure thepermeability of a polymersome to water, observations were made of thetime course for vesicle swelling in response to a step change inexternal medium osmolarity. Briefly, vesicles were prepared in thepreferred and exemplified embodiment in 100 mOsm sucrose solution toestablish an initial, internal osmolarity, after which they weresuspended in an open-edge chamber formed between cover slips, andcontaining 100 mOsm glucose. A single vesicle was aspirated into amicropipette with a suction pressure sufficient to smooth membranefluctuations. The pressure was then lowered to a small holding pressure.Using a second, transfer pipette, the vesicle was moved to a secondchamber containing 120 mOsm glucose.

When water flows out of the vesicle due to the osmotic gradient betweeninside and outside of the vesicle, the result is an increased projectionlength L_(P), which is monitored over time. The exponential decrease invesicle volume can be calculated from video images, and then fit todetermine the permeability coefficient (P_(f)) (see, e.g., Bloom et al.,1991; Needham et al., 1996). The permeability coefficient, P_(f),determined for EO₄₀-EE₃₇ was 2.5±1.2 μm/s, which, when compared withrepresentative vesicles of stearyl-oleoyl-phosphatidylcholine (SOPC)that have P_(f)=23.5±1.7 μm/s from comparable methods, indicates asignificant reduction in the permeability of the polymersomes.

The reduced permeability results mainly from the increased hydrophobicthickness. On a per area basis, EO₄₀-EE₃₇ membranes and phospholipidmembranes were found to exhibit similar fluctuations in area asunderstood from the fact that the membranes have a comparable areaexpansion modulus. Consequently, the ratio of permeabilities largelyreflects the relative probability for water to diffuse across themembrane, and the ratio of diffusion times should decrease with relativethickness of the hydrophobic core as exp(−d_(OE7)/d_(lipid)). Forpolymersomes of EO₄₀-EE₃₇, this yields exp(−8 nm/3 nm)=0.07, which is avalue close to the measured ratio of permeabilities for thesepolymersomes versus phospholipid vesicles.

The cross-linked membrane is also permeable to water. Observed volumechanges due to an osmolarity difference between the inside and outsideof cross-linked polymersomes are very similar to the volume changes ofuncross-linked vesicles under the same conditions, suggesting that thepermeability of the cross-linked membrane is quite similar to themeasured value for the exemplified EO₄₀-EE₃₇ membranes. In addition,cross-linked vesicles can be completely dehydrated in air, without lossof solutes, and rehydration leads to swelling by water permeationthrough the membrane.

(2) Permeability of the Polymersome to Encapsulated Materials

To verify the wide range of molecules encapsulated in the polymersomes,as described above, a method was devised using phase contrast microscopyto give rise to different intensities for materials with distinctoptical indices, such as sucrose and phosphate buffered saline. Nonoticeable change was detected in the intensities or the differencesbetween intensities over time periods from minutes to a month (FIG. 5B).The same was true for the intensities of fluorescently-labeled materialsin fluorescent microscopy experiments. Therefore, the polymersomemembrane is essentially impermeable to the encapsulated molecules. Theimpermeability of the cross-linked membrane was also confirmed by thefinding that these vesicles retain their encapsulated sucrose,observable through phase contrast, even after complete dehydration andrehydration of the vesicle (FIG. 11), or after 30 minute exposure tochloroform (FIG. 10).

F. Stability of Polymersomes

(1) Stability in Physiological Buffers

FIG. 5B demonstrates the long-term stability of EO₄₀-EE₃₇ polymersomesin phosphate buffered saline. Polymer vesicles were suspended in PBS,and their concentration estimated by counting the intact vesicles usinga hemocytometer at different time points. At the same time, the size ofthe vesicles was determined as an average of twenty randomly selectedvesicles. No significant change in the concentration or sizedistribution of the polymersomes was observed over period of more thanone month. Moreover, addition of ethanol to PBS had no significanteffect on the polymersome concentration or size distribution, suggestingthat such treatments can be use as sterilizing agents (TABLE 3).

(2) Thermal Stability

As shown in TABLE 4, however, the thermal stability of EO₄₀-EE₃₇vesicles was severely tested when the vesicles were exposed to autoclavetemperatures and pressures (121° C., at 2 atm) for 15 minutes. Somevesicles maintained a phase contrast and could be counted as largelyretaining their contents. At the dilute polymersome concentrations usedin these studies, the results clearly show that a significant fraction(about 10%) of polymersomes can survive a sterilizing treatment such asautoclaving. TABLE 4 Tabulation of phase dense vesicles afterautoclaving Before Autoclave After Autoclave size Size No. of vesiclesdistribution No. of vesicles distribution Trial # 10⁴/ml (μm) 10⁴/ml(μm) 1 82.4 7.3 ± 4.8 8.1 3.7 ± 0.4 2 94.3 6.0 ± 2.8 11.9 4.0 ± 0.6 3120.6 8.2 ± 5.2 10.7 3.8 ± 0.5

FIG. 5A shows the thermal stability of EO₄₀-EE₃₇ vesicles, indicatingthe membrane's area expansion with increasing temperature, and itsstability at 37° C., when the vesicle is held at a fixed membranetension of less than 4 mN/m. The relative polymer vesicle area, α, isshown against temperature. The overall thermal expansivity isapproximately 1.9×10⁻³ per degree C.

To confirm the thermal stability of the cross-linked polymersomes, theexemplified cross-linked EO₂₆-PD₄₆ vesicles containing an encapsulated250 mOsm sucrose solution were suspended in 250 mOsm glucose solution.About 0.5 ml of the vesicular solution was added to an Eppendorf testtube and submerged into boiling water for 15 minutes. The number ofvesicles before and after boiling was quantified with hemocytometer, andthe numbers were found to remain constant at the original level of10⁵/ml. Thus, the cross-linked EO₂₆-PD₄₆ vesicles are thermally stableat 100° C. for at least 15 minutes. Moreover, the increase intemperature to 100° C. did not alter the phase contrast image of theencapsulated sucrose, confirming that the impermeability of thepolymersome membrane is retained at temperatures as high as 100° C.

(3) Stability in Organic Solvents

To confirm the stability of the polymersomes in organic solvents, theexemplified cross-linked EO₂₆-PD₄₆ vesicles were inserted into one ofthe copolymer's best solvents, chloroform, and observed. Insertion ofvesicles into a droplet of chloroform carefully placed in themicromanipulation chamber altered neither the vesicle's size, nor itsshape, and the vesicle membrane remained stable for as long as it waskept in the solvent (up to 30 minutes) (FIG. 10). Small, scatteringobjects appeared inside the cross-linked vesicles when they were placedin contact with chloroform (FIGS. 10B and 10C). However, the particlesdisappeared when the vesicle was returned to aqueous solution (FIG.10D). The scattering objects simply indicate, most likely, a finitepermeability of the membrane to chloroform and formation of anencapsulated chloroform-in-water microemulsion. Moreover, examination ofthe vesicles under phase contrast microscopy directly confirmed thatthey retain large solute molecules, such as sucrose, which also has asignificant solubility in chloroform (approximately millimolar).

By contrast, uncross-linked vesicles ruptured, even before they could betransferred by micropipette into the chloroform droplet. This is becausethe small solubility of chloroform in water (about 0.5% by volume) leadsto a concentration gradient near the interface, and even this smallchloroform concentration several microns away from the interface, issufficient to selectively disrupt an uncross-linked vesicle.

(4) Stability to Dehydration and Rehydration

An additional stability test was conducted to confirm the remarkablestability of the cross-linked polymersomes to dehydration. Due to thenon-zero permeability of the cross-linked EO₂₆-PD₄₆ vesicles to water,these vesicles can be completely dehydrated in a test tube. The dryvesicles can be stored in air at room temperature for more than 24 hoursand then rehydrated by addition of water to restore the vesicle to itsoriginal volume. No noticeable difference between the original andrehydrated vesicles was been found.

Individual vesicles can be also aspirated into a micropipette and pulledfrom aqueous solution into the open air (FIG. 11). As the waterevaporates, the volume of the vesicle decreases, and the membranecollapses. The semi-dehydrated vesicle can be inserted back into aqueoussolution and rehydrated to its original shape. Phase contrast microscopyconfirmed that the encapsulated material, such as sucrose, remainsinside the dry vesicles. Therefore, the vesicles can be used inapplications that require long-term storage of material.

It is clear from the foregoing, that polymersomes are particularlyuseful for the transport (either delivery to the bulk or removal fromthe bulk) of hormones, proteins, peptides or polypeptides, sugars orother nutrients, drugs, medicaments or therapeutics, including genetictherapeutics, steroids, vitamins, minerals, salts or electrolytes,genes, gene fragments or products of genetic engineering, dyes,adjuvants, biosealants and the like. In fact, the stable vesiclemorphology of the polymersome may prove particularly suited to thedelivery of biosealants to a wound site. In bioremediation, thepolymersomes could effectively transport waste products, heavy metalsand the like. In electronics, optics or photography, the polymersomescould transport chemicals or dyes. Moreover, these stable polymersomesmay find unlimited mechanical applications including insulation,electronics and engineering.

In addition, the polymersome vesicles are ideal for intravital drugdelivery because they are biocompatible; that is they contain no organicsolvent residue and are made of nontoxic materials that are compatiblewith biological cells and tissues. Thus, because they can interact withplant or animal tissues without deleterious immunological effects, anydrug deliverable to a patient could be incorporated into a biocompatiblepolymersome for delivery. Adjustments of molecular weight, compositionand polymerization of the polymer can be readily adapted to the size andviscosity of the selected drug by one of ordinary skill in the art usingstandard techniques.

Additional encapsulation applications that involve incorporation ofhydrophobic molecules in the bilayer core include, e.g., alkyd paintsand biocides (e.g., fungicides or pesticides), obviating the need fororganic solvents that may be toxic or flammable. Polymersomes alsoprovide a controlled microenvironment for catalysis or for thesegregation of non-compatible materials.

The vesicles of the present invention further provide useful tools forthe study of the physics of lamellar phases. At different temperaturesor reduced volumes (achieved by deflating the vesicle interior with anexternal high salt solution), such vesicles will display a variety ofshapes. The formation of these shapes is dictated by the minimization ofenergy of deformation of the vesicle, namely the curvature and areaelasticity of the membrane. In fact, a series of theoretical models,called “area-difference elasticity” (ADE) models, have been used topredict a limited spectrum of different shapes seen with vesicles, suchas buds, pear-shaped vesicles and chains. Comparison between observedshapes and theoretical calculations are used to verify theoreticalconcepts of how lamellar phases behave, e.g., features such as thecurvature, or the tendency of molecules to “flip-flop” betweenmonolayers.

In addition, polymersomes have a small negative buoyancy making themsubject to gravitational shape deformations. Therefore, polymersomesafford interesting models for studying the effects of gravitation, orthe lack thereof.

The present invention is further described in the following examples.These examples are not to be construed as limiting the scope of theappended claims.

EXAMPLES Example 1 Polymersomes from Amphiphilic Diblock Copolymers

Membranes assembled from a high molecular weight, synthetic analog (asuper-amphiphile) are exemplified by a linear diblock copolymerEO₄₀-EE₃₇. This neutral, synthetic polymer has a mean number-averagemolecular weight of about 3900 g/mol mean, and a contour length ˜23 nm,which is about 10 times that of a typical phospholipid acyl chain (FIG.1A). The polydispersity measure, M_(w)/M_(n), was 1.10, where M_(w) andM_(n) are the weight-average and number-average molecular weights,respectively. The PEO volume fraction was f_(EO)=0.39, per TABLE 1.

Adapting the electroformation methods of Angelova et al., 1992, a thinfilm (about 10 nm to 300 nm) was prepared. Giant vesicles attached tothe film-coated electrode were visible after 15 to 60 min. These weredissociated from the electrodes by lowering the frequency to 3 to 5 Hzfor at least 15 min and by removing the solution from the chamber into asyringe. The polymersomes were stable for at least month if kept in vialat room temperature. The vesicles also remained stable when resuspendedin physiological saline at temperatures ranging from 10E to 50EC.

Images were obtained with a JEOL 1210 at 120 kV using a nominalunderfocus of 6 μm and digital recording. Imaging of the hydrophobiccores of these structures revealed a core thickness d=8 nm, which issignificantly greater than d=3 nm for phospholipid bilayers as describedin the Handbook of Biological Physics, 1995.

Thermal undulations of the quasi-spherical polymersome membranesprovided an immediate indication of membrane softness (FIG. 2A).Furthermore, when the vesicles were made in the presence of either a10-kD fluorescent dextran (FIG. 2B), sucrose or a protein, such asglobin, the probe was found to be readily encapsulated and retained bythe vesicle for at least several days. The polymersomes further provedhighly deformable, and sufficiently resilient that they could beaspirated into micrometer-diameter pipettes (FIGS. 2C and 2D). Themicromanipulations were done with micropipette systems as describedabove and analogous to those described by Longo et al., 1997 and byDischer et al., Science 266:1032 (1994).

The elastic behavior of a polymersome membrane in micropipetteaspiration (at ˜23° C.) appeared comparable in quality to a fluid-phaselipid membrane. Analogous to a lipid bilayer, at low but increasingaspiration pressures, the thermally undulating polymersome membrane wasprogressively smoothed, increasing the projected area logarithmicallywith tension, τ, (FIG. 3A). From the slope of this increase (in tensionunits of mN/m) versus the fractional change, α, in vesicle area thebending modulus, K_(b), was calculated (see, e.g., Evans et al., Phys.Rev. Lett. 64:2094 (1990); Helfrich et al., Nuovo Cimento D3:137(1984)).K _(b) ≈k _(B) T1n(τ)/(8πα)+constant   (1)When calculated, it was found to be 1.4±0.3×10⁻¹⁹ Joules (J), based uponthe measurements of six vesicles. In equation 1, k_(B) is Boltzmann'sconstant and T is an absolute temperature. Above a crossover tension,τ_(x), an area expansion modulus, K_(a), was estimated withK _(a)=τ/α  (2)applied to the slope of the aspiration curve as illustrated in FIG. 3.

Aspiration in this regime primarily corresponds to a true, as opposed toa projected, reduction in molecular surface density, and for thepolymersome membranes, K_(a)=120±20 mN/m (based upon 21 vesicles).Fitted moduli were checked for each vesicle by verifying that thecrossover tension, τ_(x)=(K_(a)/K_(b))(k_(B)T/8π), (Evans et al., 1990)suitably fell between appropriate high-tension (membrane stretching) andlow-tension (membrane smoothing) regimes.

Measurements of both moduli, K_(a) and K_(b), were further found toyield essentially unimodal distributions with small enough standarddeviations (approximately 20% of mean) to be considered characteristicof unilamellar polymer PEO-PEE vesicles. Interestingly, the moduli arealso well within the range reported for various pure and mixed lipidmembranes. SOPC (1-stearoyl-2-oleoyl phosphatidylcholine) in parallelmanipulations was found, for example, to be approximately K_(a)=180 mN/m(FIG. 3B) and K_(b)=0.8×10⁻¹⁹J. Lastly, at aspiration rates whereprojection lengthening was limited to <1 μm/s, the microdeformationproved largely reversible, consistent again with an elastic response.

The measured K_(a) is most simply approximated by four times the surfacetension, γ, of a pure hydrocarbon-water interface (γ=20 to 50 mJ/m²),and thus reflects the summed cost of two monolayers in a bilayer (see,e.g., Israelachvili, in Intermolecular and Surface Forces, 2^(nd) ed.,Sec. III, 1995). The softness of K_(a) compared with gel or crystallinestates of lipid systems is further consistent with liquid-like chaindisorder as described by Evans et al., 1987. Indeed, because the averageinterfacial area per chain, <A_(c)>, in the lamellar state has beenestimated to be <A_(c)>/2.5 nm² per molecule ( see, e.g., Hajduk et al.,1998; Warriner et al., Science 271: 969 (1996); Yu et al., 1998), theroot-mean-squared area fluctuations at any particular height within thebilayer can also be estimated to be, on average, <δA_(c)²>^(1/2)=(<A_(c)>k_(B) T/K_(a))²/0.3 nm² per molecule, which is asignificant fraction of <A_(c)> and certainly not small on a monomerscale.

Moreover, presuming in the extreme, a bilayer of unconnected monolayersd/2 thick, with d estimated from cryo-TEM (FIG. 1), the PEE contourlength is more than twice the monolayer core thickness, and therefore,configurationally mobile along its length. In addition, moleculartheories of chain packing in bilayers have suggested that, although at afixed area per molecule there is a tendency for K_(b) to increase withchain length (that is, membrane thickness), other factors such as large<A_(c)> can act to reduce Kb (see, e.g., Szleifer et al., Phys. Rev.Lett. 60:1966 (1988); Ben-Shaul, in Structure and Dynamics of Membranesfrom Cells to Vesicles, in Handbook of Biological Physics, vol. 1, chap.7 (Elsevier Science, Amsterdam, 1995)). Thus, despite the large chainsize of EO₄₀-EE₃₇, a value of K_(b) similar to that of lipid bilayers isnot surprising.

Related to the length scales above, the root ratio of moduli,(K_(b)/K_(a))^(1/2), is generally recognized as providing aproportionate measure of membrane thickness (see, e.g., Handbook ofBiological Physics, supra; Bloom et al., 1991; Needham et al., 1996,chap. 9; and Petrov et al., Prog. Surf. Sci. 18:359 (1984)). For thepresently described polymersome membranes, (K_(b)/K_(a))^(1/2)=1.1 nm onaverage. By comparison, fluid bilayer vesicles of phospholipids orphospholipids plus cholesterol, have reported a ratio of(K_(b)/K_(a))^(1/2)=0.53 to 0.69 nm (Evans et al., 1990; Helfrich etal., 1984). Typically, the fluid bilayer vesicles of phosholipids pluscholesterol have a higher K_(a) than those of phospholipid alone.

A parsimonious continuum model for relating such a length scale tostructure is based on the idea that the unconnected monolayers of thebilayer have, effectively, two stress-neutral surfaces located near eachhydrophilic-hydrophobic core interface (see e.g., Petrov et al., Prog.Surf Sci. 18:359 (1984)). If one assumes that a membrane tensionresultant may be located both above and below each interface, then(K _(b) /K _(a))=δ_(H)δ_(C)   (3)where δ_(H) and δ_(C) are, respectively, distances from the neutralsurfaces into the hydrophilic and hydrophobic cores.

For lipid bilayers with d/2=1.5 nm and hydrophilic head groups equal to1 nm thick, estimates of δ_(C)=0.75 nm and δ_(H)=0.5 nm yield aroot-product, (δ_(H)δ_(C))^(1/2)=0.61 nm. This is consistent withexperimental results. The numerical result for PEO-PEE membranes (1.1nm) suggests that the stress resultants are centered further from theinterface, but not necessarily in strict proportion to the increasedthickness or the polymer length.

Elastic behavior terminates in membrane rupture at a critical tension,τ_(c), and areal strain, a α_(c). With lipids, invariably α_(c)=0.05.This is consistent, it appears, with a molecular theory of membranesunder stress (see, e.g., Netz et al., Phys. Rev. E 53:3875 (1996)describing self-consistent calculation models of lipids). For thepolymersomes, cohesive failure occurred at α_(C).=0.19±0.02 (FIG. 3B).

Another metric is the toughness or cohesive energy density that, forsuch a fluid membrane, is taken as the integral of the tension withrespect to area strain, up to the point of failure:E _(c)=½K _(a)α_(c) ²   (4)

For a range of natural phospholipids mixed with cholesterol, thetoughness has been systematically measured, with E_(c) ranging from 0.05to 0.5 mJ/m² (see, Needham et al., 1990). By comparison, the EO₄₀-EE₃₇membranes are 5 to 50 times as tough, with E_(c)≈2.2 mJ/m². On a permolecule, as opposed to a per area basis, such critical energies areremarkably close to the thermal energy, k_(B)T, whereas such an energydensity for lipid bilayers is a small fraction of k_(B)T. Thisdifference indicates, that for this relatively simple condensed mattersystem, the strong role that fluctuations in density have in creating alytic defect.

Despite the comparative toughness of the polymersome membrane, a core“cavitation pressure,” pc, may be readily estimated as:p _(c)=τ_(c) /d   (5)yielding a value of p_(c)=−25 atm. This value falls in the middle of therange noted for lipid bilayers, p_(c)=−10 atm to −50 atm (see, e.g.,Bloom et al., 1991; Needham et al., 1996). Bulk liquids, such as waterand light organics, are commonly reported to have measured tensilestrengths of such a magnitude, as may be generically estimated from aratio of nominal interfacial tensions to molecular dimensions (that is,˜γ/d). In membrane systems, this analogy again suggests an importantrole for density fluctuations, which are manifested in a small K_(a),and which must become transversely correlated upon coalescing into alytic defect.

Because the previous estimate for <δA_(c) ²>^(1/2) is clearly not smallas compared with the cross section of H₂O, a finite permeability of thepolymersome membranes to water was expected. To verify this expectationpolymersome permeability was obtained by monitoring the exponentialdecay in EO₄₀-EE₃₇ vesicle swelling as a response to a step change inexternal medium osmolarity. Vesicles were prepared in 100 mOsm sucrosesolution to establish an initial, internal osmolarity, after which theywere suspended in an open-edge chamber formed between cover slips andcontaining 100 mOsm glucose. A single vesicle was aspirated with asuction pressure sufficient to smooth membrane fluctuations; after whichthe pressure was lowered to a small holding pressure.

With a second, transfer pipette, the vesicle was moved to a secondchamber with 120 mOsm glucose. Water flowed out of the vesicle due tothe osmotic gradient between the inner and outer surfaces, which led toan increased projection length that was monitored over time. Theexponential decrease in vesicle volume was calculated from video images,and then fit to determine the permeability coefficient (P_(f)) (see,e.g., Bloom et al., 1991; Needham et al., 1996). The permeabilitycoefficient, P_(f), was 2.5±1.2 μm/s.

In marked contrast, membranes composed purely of phospholipids with acylchains of approximately 18 carbon atoms typically have permeabilities inthe fluid state at least an order of magnitude greater (25 to 150 μm/s).Polymersomes are thus significantly less permeable to water, whichsuggests beneficial applications for the polymersomes.

Example 2 Crosslinked Polymersomes

Given the flexibility of copolymer chemistry, the stealth character aswell as the cell stability can be mimicked with amphiphilic diblockcopolymers that have a hydrophilic fraction comprising PEO, and ahydrophobic fraction which can be covalently cross-linked into anetwork. One example of a diblock copolymer having such properties,along with the capability of forming several morphologically differentphases, is polyethylene oxide-polybutadiene (PEO-PBD).

EO₂₆-BD₄₆, spontaneously forms giant vesicles as well as smallervesicles in aqueous solutions without the need of any co-solvent.Cross-linkable unilamellar vesicles were fabricated. The formed vesicleswere cross-linked by free radicals generated with a of initiating K₂S₂O₈and a redox couple Na₂S₂O₅/FeSO₄.7H₂O as described above. When theosmolarity of the cross-linking reagents was kept the same as that ofthe vesicle solution, neither addition of the cross-linking reagents northe cross-linking reaction itself affected vesicle shape.

Osmotically inflated vesicles remained spherical, independent of thecross-linked state of the membrane (FIGS. 9A and 9C). Consequently, thefully inflated spheres, pearls of interconnected spheres, and othershapes appeared unchanged from the way they were observed prior to thecross-linking reaction. When fluid phase vesicles are osmoticallydeflated, the result is a flaccid shape, with a smooth contour (FIG.9B). However, when the cross-linked vesicles were osmotically deflatedafter the cross-linking reaction was completed, the vesicles revealedthe solid character of the membrane—with irregularly deformed creasedstructures (FIG. 9D). The difference reflected the fact that, whenexposed to a change in osmolyte, the cross-linked molecules could notsignificantly rearrange within their surface to relax the accumulatedstrain.

The cross-linked EO₂₆-BD₄₆ vesicles were initially tested for stabilityby direct observation of the vesicles inserted into a solvent,chloroform. However, chloroform altered neither the size, nor the shapeof the vesicles, and the vesicle membrane remained stable for as long asit was kept in the solvent. The mechanical properties of the vesiclewhen exposed to solvent are shown in FIG. 10. FIG. 10A depicts a vesiclein aqueous solution being pulled into a micropipette by negativepressure, ΔP. FIG. 10B depicts the same vesicle imaged immediately afterbeing placed into chloroform. After 30 minutes exposure to chloroform,there was no noticeable change observed in the vesicle (FIG. 10C); andthe vesicle remained unchanged after it was returned to the aqueoussolution (FIG. 10D).

If a significant portion (few weight percent) of the solutes were lostfrom the vesicle during chloroform exposure, the aspirated projection ofthe vesicle would have lengthened. However, no detectable changeoccurred in either surface area or volume. This demonstrated that thecross-linked membrane maintains its integrity when exposed to organicsolvent. By comparison, uncross-linked vesicles cannot be exposedwithout rupture to aqueous solutions containing a saturatingconcentration of solvent (approximately 0.8 g/dl chloroform).

A second stability test was based upon complete dehydration. Due to thefinite water permeability of the cross-linked vesicles, they can becompletely dehydrated in a test tube. Dry vesicles were stored in air,at room temperature, for more than 24 hours, then rehydrated by theaddition of water to their original volume. However, no noticeabledifference between the original and rehydrated vesicles was found.

Individual cross-linked vesicles were also aspirated into amicropipette, pulled from the aqueous solution (FIG. 11A) and exposed tothe open air (FIG. 11B). As the water evaporated and the vesicledehydrated, the volume decreased, and the membrane crinkled.Nevertheless, when the semi-dehydrated vesicle was returned to theaqueous solution, it was immediately rehydrated to its original shape(FIG. 11C). Within 1 minute of rehydration, the original shape of thedehydrated vesicle was almost completely restored, indicating theretention of solutes within the vesicle. Phase contrast microscopyfurther confirmed that encapsulated material, such as sucrose, remainedinside the dry vesicles. Therefore, the cross-linked vesicles can beused in applications that require long-term storage of material.

To finally confirm the stability of the cross-linked vesicles,deformation tests were done by micropipette manipulation (FIG. 12). Themaximum applied aspiration pressure in the experimental setup, ΔP=1 atm,did not lead to rupture of the cross-linked vesicles. Since the typicalmicropipette radius in the experiment was 4 μm, such high pressures ledto membrane tension at the cap, τ=½ ΔPRP of around 200 mN/m, which is anorder of magnitude higher than the lysis tension of red blood cells. Atypical aspiration curve of a flaccid, nearly spherical (but notpressurized) vesicle is shown in FIG. 12A. Such aspiration curves can bedone repeatedly, indicative of the membrane's elasticity.

Since the aspirated vesicles were flaccid, but almost spherical andnon-pressurized, it was assumed that during initial aspiration, the areaof the vesicle is constant, and that the bending becomes negligible withrespect to shearing of the membrane. Given those assumptions, computersimulations for the shearing of the vesicle in the pipette indicatedthat the shear modulus is between one and two times the slope ofτ/(L/R_(p)) versus R_(v)/R_(p) (FIG. 12B). This was equal to about 150mN/m, which is four orders of magnitude higher than the shear modulus ofred blood cells, which was determined to be about 0.01 mN/m.

Although proving that a membrane is completely cross-linked is not atrivial task, and controversy is often associated with the subject, thestability tests reported in the present example provide the best directevidence to date to confirm complete cross-linking. Cross-linkingreactions introduce local stresses in the membrane, making it moredifficult to completely cross-link a large (cell-size) structure that isself-assembled from monomers with a limited number of cross-linkableentities. However, by expanding the size of the polymerizable block inthe present invention, the difficulties have been overcome.

Example 3 Polymersomes from Amphiphilic Triblock andMulti-BlockCopolymers

Multi-block copolymers offer an alternative approach to modifying theproperties of the polymersome. Insertion of a middle B block in atriblock copolymer permits modification of permeability and mechanicalcharacteristics of the polymersome without chemical cross-linking. Forexample, if the B and C blocks are strongly hydrophobic, yet mutuallyincompatible, and the A block is water miscible, two segregated layerswill form within the core of the membrane. This configuration ofinterfaces (internal B-C and external B-hydrated A) offers control ofthe spontaneous curvature of the membrane among other features such asheight-localized cross-linking. Thus, vesicle size will depend, in part,on block copolymer composition. Of course, as noted above, the physicalproperties of the ABC polymersome will reflect a combination of the B, Cand hydrated A mechanical behaviors. An example of such a triblockcopolymer, which does form vesicles is EO₃₃-S₁₀-I₂₂ (TABLE 1), whereinEO is polyethyleneoxide, S is styrene, and I is isoprene.

Another arrangement for the triblock, which would form vesicles, is ABAor ABC wherein A and C are water miscible blocks and B is thehydrophobic block. In such case the copolymer can self-assemble in“straight” form into a monolayer or in “180° bent” form into a bilayer,or as a combination of these two forms. An example of this kind of ABAtriblock, which does form vesicles, is EO₄₈-EE75-EO48 (TABLE 1).

Example 4 Vesicles of Mixed Composition

Vesicles comprising diblock copolymer mixtures have been prepared by themethods described above for a wide ratio of diverse amphiphiliccomponents. As a first example, mixture of cross-linkable diblockcopolymers with noncross-linkable ones can be made. However, in contrastto the stabilizing effect of cross-linking on vesicles fabricated frompurely cross-linkable amphiphiles as described above, the dilution ofcross-linkable amphiphiles with non-cross-linkable molecules couldproduce a less stable membrane upon cross-linking, resulting in acontrolled-release membrane.

For the purpose of this invention, the percolation threshold is a weightfraction of the cross-linkable copolymer above which the cross-linkingreaction leads to a single cross-linked domain spanning the entirevesicle surface. Below the percolation threshold, a single cross-linkeddomain does not span the entire vesicle surface and is likely to be muchless stable than a wholly cross-linked vesicle. For example, mixtures ofEO₄₀-EE₃₇ and EO₂₆-PD₄₆ copolymers with the weight fraction of EO₂₆-PD₄₆equal to 0.5 were found to be extremely fragile after the cross-linkingreaction as compared with single component polymersome membranes (andtherefore below the percolation threshold).

Increase of the weight fraction to 0.6 caused the vesicles to be morestable than the uncross-linked membranes, but far more fragile than thevesicles composed of purely cross-linkable amphiphiles, as demonstratedby the leakage of encapsulated material (FIG. 14). Therefore,appropriate mixing of different components can be used to modulatevesicular stability. The destabilization by this type of cross-linkingreaction can be applied to controlling the release of contents from thepolymersome vesicle. Consequently, the polymersome can be induced torelease an encapsulated component, either chemically and/or by wavepropagation (such as, X-rays, UV, visible light, IR irradiation, andultrasound).

In the same way, mixtures can be made of the copolymer amphiphiles withother synthetic or non-synthetic amphiphiles, such as, lipids orproteins. For example, 3% of a Texas-Red labeledphosphatidylethanolamine preparation was incorporated into an EO₄₀-EE₃₇membrane with no obvious effect on either membrane structure or areaexpansion modulus (FIG. 6). FIGS. 6A and 6B show the uniformity offluorescence around an aspirated contour of membrane with 3 mol % mixedin with polymer before vesicle formation. The uniformity of thefluorescence can be seen around an aspirated contour of the membranedemonstrating good mixing in the membrane.

Moreover, in FIG. 6C the contour intensity was seen to increase linearlyas the concentration of Texas Red was increased to about 10 mol %,demonstrating ideal mixing of the components at that concentrationrange. Laser-photobleaching demonstrates that lipid probe diffusivity is20-fold lower on average in the polymer membrane than in a lipid (SOPC)membrane which, by the present method has a diffusivity of approximately3×10⁻⁸ cm²/s.

Based on the above features of amphiphile incorporation into polymersomemembranes, the fluorescent lipophilic probe diI(C18) has beenincorporated at a few mole percent into cross-linkable membranes andshown to yield unstable membranes after approximately 60 minutes offluorescence excitation and photobleaching.

Example 5 Capsules Formed in Emulsion

Based upon the principles presented in the present invention, emulsionsoffer unique possibilities for transporting hydrophobic materials in anaqueous medium. Thus, the invention provides for the self-assembly of asuper-amphiphile layer around an oil droplet in water, with or withoutcross-linking of the super-amphiphile. Capsules, similar to polymersomevesicles, can be formed in microemulsions, in which oil droplets aredispersed in water and the amphiphile is used to stabilize the oil-waterinterface. The preferable amount of oil is 1 to 5% (v/v) in aqueoussolution. The amphiphile can be dissolved in either the aqueous or oilphase before mixing. The mass of the added amphiphile depends on thedesired size of the emulsion as determined from the following equation:Mass(m)=3V _(T) M _(n)/(r _(v) N _(A) A),   (6)wherein V_(T) is the total volume to be encapsulated, M_(n) is molecularweight of the amphiphile, r_(v) is the radius of the microemulsiondroplets, N_(A) is the Avogadro number, and A is the molecular area ofthe amphiphile at the surface. The factor of three comes from simplegeometry.

Relatively monodisperse emulsions were prepared by adding oil to theaqueous phase and filtering several times through polycarbonate filters.The pore size of the filter controls the size of the droplets.Therefore, the size of the microemulsion is limited only by theavailability of the polycarbonate filters with desirable pore sizes.Currently, pore sizes of commercially available filters range from 0.01to 20 μm (Osmonics, Livermore, Calif., USA).

The amphiphilic molecules form a stabilizing monolayer at the oil-waterinterface. When part or all the amphiphile contains a cross-linkablemoiety, the surface of the microemulsion was efficiently cross-linked bythe generation of free radicals as described above for the cross-linkingof vesicles (FIG. 13). This cross-linking further stabilized thedroplets against their natural tendency to coalesce with other droplets.For example, the method was successfully used to stabilize 10 μmkerosene droplets in aqueous solution with a PEO-PB of molecular weightM_(n)=8,100 and hydrophilic volume fraction of 62% (see FIG. 13A,wherein PEO is shown facing the water, and PB is shown facing the oil).

After completion of the redox reaction, e.g.,(K₂S₂O₈/Na₂₂S₂O₅/FeSO₄.7H₂O as described above), the 10 μm droplets wereof a size that could be aspirated without fragmentation into amicropipette with radius R_(p)=4 μm by negative pressure, ΔP=0.200 atm(FIG. 13B). The resulting droplets of oil are well suited forencapsulation and transport of hydrophobic material, and the amphiphilecapsules prepared using the emulsion method offer exciting newopportunities as, e.g., artificial blood cells. For example,perfluorocarbon emulsions have already shown promise as oxygen-carryingblood substitutes.

In sum, polymersomes, enable direct measurements of the materialproperties of lamellae and permit characterization of membrane assembly.The preparation methods of the present invention provide additional waysto “engineer” bilayer membranes. As compared with lipids, the increasedlength and conformational freedom of polymer chains of this invention,not only provide a basis for enhanced stability, toughness and reducedpermeability of membranes, but also provide a rich diversity of blockcopolymer chemistries (molecular weights, block fraction, blockarchitecture), thereby furnishing a plethora of novel, artificialmembranes and tissues, soft biomaterials and biomimetic structures,controlled-release vehicles and systems for engineering and biomedicalapplications.

All patents, patent applications and publications referred to in thepresent specification are also fully incorporated by reference.

While the foregoing specification has been described with regard tocertain preferred embodiments, and many details have been set forth forthe purpose of illustration, it will be apparent to those skilled in theart that the invention may be subject to various modifications andadditional embodiments, and that certain of the details described hereincan be varied considerably without departing from the basic principlesof the invention. Such modifications and additional embodiments are alsointended to fall within the scope of the appended claims.

1. A vesicle comprising a semi-permeable, thin-walled encapsulating membrane, wherein the membrane is formed in an aqueous solution, and wherein the membrane comprises one or more synthetic super-amphiphilic molecules.
 2. A polymersome.
 3. The vesicle of claim 1, wherein at least one super-amphiphile molecule is a block copolymer and wherein the resulting vesicle is termed a polymersome.
 4. The polymersome of claim 3, comprising a diblock copolymer.
 5. The polymersome of claim 3, comprising a triblock copolymer.
 6. The polymersome of claim 3, wherein all of the super-amphiphile molecules are block copolymers.
 7. The polymersome of claim 3, wherein the vesicle is prepared together with one or more small amphiphiles.
 8. The polymersome of claim 7, wherein at least one small amphiphile is a phospholipid.
 9. The polymersome of claim 3, wherein the vesicle self-assembles in aqueous solution.
 10. The polymersome of claim 3, wherein at least one block copolymer is selected from the group consisting of polyethylene oxide (PEO), poly(ethylethylene) (PEE), poly(butadiene) (PB), poly(styrene) (PS) and poly(isoprene) (PI).
 11. The polymersome of claim 1, wherein the copolymers are cross-linked.
 12. The polymersome of claim 11, wherein the integrity of the membrane is maintained when the polymersome is exposed to an organic solvent or boiling water, and when the polymersome is dehydrated in air or rehydrated in an aqueous solution.
 13. The polymersome of claim 3, wherein the vesicle is biocompatible.
 14. The polymersome of claim 3, wherein the polymersome encapsulates at least one composition selected from the group consisting of a drug, therapeutic compound, dye, nutrient, sugar, vitamin, protein or protein fragment, salt, electrolyte, gene or gene fragment, product of genetic engineering, steroid, adjuvant, biosealant, gas, ferrofluid, and liquid crystal.
 15. The method of using the polymersome of claim 3 to transport an encapsulatable material to or from the environment immediately surrounding the polymersome.
 16. The method of using the polymersome of claim 13 to transport to or from a patient a composition consisting of a drug, therapeutic composition, dye, nutrient, sugar, vitamin, protein or protein fragment, salt, electrolyte, gene or gene fragment, product of genetic engineering, steroid, adjuvant, biosealant and gas to a patient in need of such composition.
 17. The method of preparing the polymersome of claim 3, comprising at least one step consisting of a film rehydrating step, a bulk rehydrating step, or an electroforming step.
 18. A method of controlling the release of an encapsulated material from a polymersome of claim 3 by modulating the composition of the membrane.
 19. A method of controlling the release of an encapsulated material from a polymersome of claim 18 by cross-linking a membrane comprising at least one cross-linkable amphiphile and at least one non cross-linkable molecule, and subjecting the thus destabilized membrane to chemical exposure or propagated light, sound, heat, or motion.
 20. An encapsulating membrane comprising a semi-permeable, thin-walled encapsulating, amphiphilic membrane, wherein the membrane is formed around a droplet of oil in a microemulsion of oil dispersed in an aqueous solution, and wherein the membrane comprises one or more synthetic super-amphiphilic molecules. 